Hot melt adhesives are applied to a substrate while in a molten state and cooled to harden the adhesive layer. Such adhesives are widely used for various commercial and industrial applications including, to seal cardboard cases, bags, trays and cartons; and to bond laminates for nonwoven and foil articles. Some packaging applications require an adhesive to maintain a strong bond to a substrate under extremes of stress and shock in handling. Moreover, cases and cartons often encounter very high temperatures during transportation, so adhesives having sufficiently good heat resistance are required in these applications. “Sufficiently good heat resistance” is to be understood to mean that the bonded adhesive maintains fiber tear at elevated temperature, e.g., greater than 52° C. (125° F.), and hence should not immediately soften when acted upon by elevated temperature, with the result that the adhesive bond loosens and/or the bonded parts shift with respect to one another.
Foamed hot melt adhesives based on various polymers are described in U.S. Pat. Nos. 5,342,858, 5,369,136, 6,008,262 and JP H5-17730. Foamed adhesives contain closed-cell foams with pockets of gas or air in the matrix of the adhesive. There are various economic and environmental benefits to using foamed adhesives, including lower usage, lower cost, lower waste, lower storage requirements, and greater gap filling capability. While the benefits of foamed adhesives are numerous, foamed adhesives typically have lower adhesion and lower reliability than the unfoamed counterpart.
The present invention seeks to improve the reliability and adhesion of foamed hot melt adhesives. In addition, the foamable hot melt adhesives and the packages made therefrom provide more environmentally and economically sound products.